The present invention relates to a method of recording information signals in digital form on a memory medium at a relatively high density or reproducing (or detecting) information signals previously recorded in digital form on a memory medium at a relatively high density, and a head device and a memory medium used for the recording/reproducing method.
Various kinds of large-capacity memories are presently known, for example, a semiconductor memory represented by a DRAM or Flash memory, a magnetic tape represented by a video recorder, and a disk memory represented by a compact disk or hard disk. These memories have problems, for example, in terms of high bit cost and low access speed and may be not suitably used for future information inputting/outputting apparatuses such as a microprocessor or network requiring a more increased data transfer rate and data capacity. A related art hard disk, optical disk or magnetic tape is lower by two digits or more in cost per unit data (bit) than a semiconductor memory; however, it is significantly inferior in access time, data transfer time and volume of the disk to the semiconductor memory.
At present, with the enhanced performances of computers and the increased communication speed of information networks, the quantity of data to be processed has become larger and the processing rate of data to be processed has become high. To meet such technical development, it is desired to realize a read only memory and a writable memory with the cost per bit kept substantially comparable to that of a magnetic disk or optical disk and with the access time, data transfer time and volume of the memory increased to the levels comparable to those of a semiconductor memory.
The size of a semiconductor chip, for example, used in a DRAM has become larger with the progressing technical generations, and it is expected that the size of a semiconductor chip is to be more than about 3xc3x973 cm at the 4 Gbit-generation. In this case, the area including a package will be about 12 cm2. To be used like such a DRAM, a memory to be developed is desired to have a size smaller than the above value and a low bit cost.
The memory capacity stored in the above-described area (about 12 cm2) is preferably equivalent to a capacity, for example, which is capable of storing dynamic images for about one hour, and more specifically, the memory capacity is required to have about 12 Gbits, that is, a memory density of at least 1 Gbit/cm2 in consideration of digital image signals with the compressed frequency band.
As a memory capable of meeting the above-described requirement, there have been extensively studied memories of a type using a so-called SPM (Scanning Probe Microscope) such as a STM (Scanning Tunneling Microscope) or AFM (Atomic Force Microscope).
Such a memory has been described in detail, for example, in xe2x80x9cH. J. Mamin et al.: IBM J. Res. Develop. Vol. 39, 681 (1995)xe2x80x9d. This memory detects an information signal using a head device 100 shown in FIG. 10A. The head device 100 has a beam cantilevered with its one end fixed on a head substrate 102, which is generally called a cantilever 103, and a head element 101 as a signal detecting portion (hereinafter, referred to simply as xe2x80x9chead elementxe2x80x9d) formed at the leading end of the cantilever 103. The head element 101 is sharply pointed into the shape of a triangular or quadrangular prism by a semiconductor process. The leading end of the head element 101, sharply pointed up to the level of atomic, size is moved close to the surface of a substance to be measured (data surface in the case of the memory), and an interatomic force acting between the head element 101 and the surface of the substance or a tunneling current flowing therebetween is directly or indirectly measured, to thus obtain information therefrom.
B. D. Terris et al. have reported in xe2x80x9cAppl. Phys. Lett. 69(27), 4262(1996)xe2x80x9d a method of preparing a data patten applicable to a disk-like medium by an electron beam plotting apparatus, transferring the patten on an ultraviolet cured resin layer formed on a glass disk by a so-called glass 2P process to prepare a data disk, and reproducing data signals stored in the disk by an AFM.
H. J. Mamin et al. have reported in xe2x80x9cSensors and Actuators A48, 215(1995)xe2x80x9d a method of bringing a leading end of an AFM in contact with a high polymer substrate, heating the leading end of the AFM by laser to melt the surface of the high polymer substrate, thereby recording data on the surface of the high polymer substrate, and reproducing the data at a reproducing rate as high as 1 Mbit/sec by the AFM.
The apparatus for recording/reproducing information signals in each of these documents carries out recording/reproducing with the disk medium rotated using one AFM head.
H. Kado et al. have reported in xe2x80x9cAppl. Phys. Lett. 66(22), 2961(1995)xe2x80x9d a method of forming a platinum thin film on a silicon substrate and also forming an amorphous GeSb2Te4 film thereon, carrying out recording by applying a pulsive electric field between a sharpened conductive head element and the platinum thin film, and carrying out reproducing by detecting a difference in electric conductivity as a change in current.
The above detection of data using the AFM, however, is not suitable for reproducing information signals at a high rate because the interatomic force is converted into a mechanical displacement of the cantilever 103 and the displacement is detected by a displacement meter using a piezoelectric effect or laser. Also in the case where information signals are recorded or reproduced on or from a rotating disk-like recording medium, the above detection of data using the AFM is disadvantageous in that it takes a time to wait rotation and the access speed becomes low.
To improve an effective data transfer rate, there has been known a method of carrying out parallel processing using a plurality of head devices. For example, S. C. Minne et al. have reported in xe2x80x9cAppl. Phys. Lett. 67(26), 3918(1995)xe2x80x9d an apparatus in which two AFM head devices with leading ends of head elements separated 100 xcexcm from each other are arranged in parallel whereby images of a grating with a cycle of 5 xcexcm are reproduced. In this parallel processing apparatus, ZnO having a piezoelectric effect is used for part of each cantilever to individually displace the two cantilevers in the depth direction, and the size of the cantilever becomes larger (length: 420 xcexcm, width: 85 xcexcm) for sufficiently ensuring the displacement, with a result that the mechanical resonance frequency becomes low to reduce the data transfer rate. Accordingly, even in the case of using a plurality of the AFM head devices, the data transfer rate is not improved so much.
The data reproducing apparatus (microscope) using a plurality of the head elements described in the above document does not report a method or mechanism of detecting or correcting a positional relationship between each head element and desired data in the direction parallel to the surface to be measured, causing a problem that the address management for data which is important for the memory apparatus cannot be performed. Even if each distance and positional relationship between two pieces of respective head elements has been clearly measured and also the positional relationship between either one of the head elements and the address of the data surface has been measured, the relationship between each head element and the data position cannot be kept resulting from a difference in thermal expansion coefficient between the head array and the substrate of the recording medium, for example, caused by temperature change.
In the memory apparatus using the SPM, since the leading end of the head element 101 of the head device 100 is very sharpened as shown in FIG. 10B and only the leading end of the head element 101 is brought in contact with the data surface, if an impact force is applied to the memory apparatus during reproducing of data, the data surface in contact with the leading end of the head element 101 may be applied with a very high local pressure, which causes a fear of destruction of data stored in the data surface. For example, S. C. Minne et al. have reported in xe2x80x9cSensors and Actuators A48, 215 (1955)xe2x80x9d that a leading end of a head element is formed into a spherical shape having a curvature of 100 nm or less. A spring constant of the cantilever is in the order of 1 N/m. Now, it is assumed that a leading end of a head element applied with an impact force is displaced 10 nm on the data surface side and is brought in contact with the data surface; and the leading end of the head element is formed into a flat circular shape having a radius of 10 nm. In this case, a pressure applied to the flat circle portion is as very high as 3xc3x97107 N/m2, which will cause destruction of data stored on the data surface of a medium, insofar as the medium is made from a usual material. Even if there is not a destruction of data, since the leading end of the head element is worn, there arises another problem that the shape of the leading end is changed and thereby the resolution in recording or reproducing is reduced.
In the above-described recording/reproducing method proposed by H. Kado, since a current value upon reproducing is 10 pA at a location where information signals have not been recorded and is 1 nA at a location where information signals have been recorded, a reproducing signal with a sufficient S/N can be obtained when the data reproducing rate is low because the frequency band of the reproducing signal is narrow; however, a reproducing signal with a sufficient S/N cannot be ensured at the above small signal current of about 1 nA when the data reproducing rate is high because the frequency band of the reproducing signal becomes wide.
An object of the present invention is to provide a method of recording/reproducing information signals on/from a ROM and a writable memory medium at an access speed in the order of xcexcs, a recording density of 1 to 10 GBs/cm2, and a data transfer rate in the order of Gbit/sec without destruction of the data.
To achieve the above object, according to the present invention, there is provided a method of recording/reproducing an information signal, including the steps of: making a head device face to a memory medium having a flat recording surface, the head device including a plurality of head elements two-dimensionally arranged each of which has at its leading end a flat portion having an area of 0.1 xcexcm2 or less; moving the head device relative to the memory medium a distance more than a gap between two adjacent ones of the head elements; and recording an information signal at a specific position of the recording surface at a recording density of 1 Gbit/cm2 or more, or reproducing an information signal previously recorded on the recording surface at a specific position by the head device.
As described above, according to the method of and apparatus for recording/reproducing information signals in accordance with the present invention, there can be obtained a ROM system and a writable memory system having an access speed in the order of xcexcs, a recording density of 1 to 10 GBs/cm2, and a data transfer rate in the order of Gbit/s.
The information signal recording/reproducing apparatus of the present invention is inferior in processing rate and reliability to a semiconductor memory but is lower two digits or more in bit cost than the semiconductor memory, and therefore, it can be applied to a variety of technical fields. That is to say, the recording/reproducing apparatus of the present invention may be used in applications including a CD, MD, electronic game machine, DVD and video camera, in place of an optical disk and a magnetic disk, particularly, it may be effectively used for a portable CD player called a walkman, digital camera or may be used in place of a hard disk for a laptop type computer or the terminal of a portable computer.
The method of and apparatus for recording/reproducing information signals in accordance with the present invention are also very effectively used for applications which require processing of a large amount of data, high speed retrieval, and high speed access, for example, for a recognition data base for video recognition and image recognition and a data bank for video demand distribution.